TRIPLE ONLY: Alcohols

What are alcohols?

A homologous series of compounds which all contain an —OH functional group covalently bonded to a hydrocarbon chain

Describe the ways to oxidise ethanol + their products

• Complete combustion: burning in air or oxygen to form CO2 and H2O

• Microbial oxidation: reaction with oxygen in the air to form ethanoic acid (vinegar) with the help of microorganisms such as bacteria or yeast

• Heating (in water bath) with potassium dichromate(IV) in dilute sulfuric acid to form ethanoic acid

The most common way of oxidising ethanol is heating with potassium dichromate(IV) in dilute sulfuric acid to form ethanoic acid.

What is the oxidising agent? Why is dilute sulfuric acid used?

A mixture of potassium dichromate(VI) and dilute sulfuric acid

It is important for the potassium dichromate(VI) to act as an oxidising agent. Without the H+ ions from the acid, no redox reaction will occur

Colour change in the oxidation reaction of ethanol by heating with potassium dichromate(VI) and dilute sulfuric acid?

What is this colour due to?

Turns from orange to green

• Orange = ethanol + potassium dichromate(VI) + dilute sulfuric acid

• Green = indicates the presence of Cr³⁺ ions, which are formed from the reduction of potassium dichromate(VI)

Displayed formula equation for ethanol + potassium dichromate(VI) and dilute sulfuric acid?

Whatever the flip this apparatus is

Ethanol is a biofuel. What is meant by the term biofuel?

A fuel that is made from biological sources, such as sugar can or corn

Why are biofuels used in countries?

Countries that have little or no oil industry to produce their own petrol.

Countries that have a climate which is suitable for growing sugar caneç

Countries that want to reduce dependence on fossil fuels, which are finite non-renewable resources

What are the two different processes that are used industrially to make ethanol?

• Fermentation

• Hydration

Making ethanol by fermentation

Process/details?

• Requires aqueous glucose as the reagent

• Leave the solution at 30°C at 1 atm for several days in anaerobic conditions

• The catalysts are the enzymes in the yeast which convert the sugar into ethanol and carbon dioxide

Equation for fermentation to obtain ethanol

C₆H₁₂O₆ (aq) → 2C₂H₅OH (aq) + 2CO₂ (g)

Why is the absence of air and the temperature important?

• Anaerobic conditions

  • In the presence of air, enzymes in the yeast produce carbon dioxide and water instead of ethanol

• Temperature

  • The enzymes are proteins and if temp is too high, then they will denature so active site changes shape and substrate no longer fits

  • At lower temperatures, the reaction is too slow

Why is it impossible to make pure alcohol by fermentation?

What process is needed to obtain pure alcohol from fermentation? How does this work?

Yeast is killed by more than about 15% of alcohol in the mixture.

The alcohol has to be purified by fractional distillation. This takes advantage of the difference in boiling point between ethanol and water.

Making ethanol by hydration

Process/details? Equation?

• The reagents are ethene and steam

• Temperature: 300

• Pressure: 60-70 atm

• Catalyst: phosphoric acid (H₃PO₄)

Only a small proportion of the ethene reacts. Why might this be?

Because of this, the end product we get is ethanol, ethene and steam. So what do we do?

It is a reversible reaction (segun Copilot)

• Ethanol has a much higher boiling point than ethene, so when the mixture is cooled:

  • ethanol condenses to a liquid

  • ethene stays as a gas

  • This makes it easy to separate them.

• The unreacted ethene is recycled and used again for the process

Key point:

What kind of reaction is this? Why?

This is an addition reaction because the water is added to the ethene without removing anything

Compare the two methods of producing ethanol